Heat exchanger assembly and system for a cryogenic air separation unit

ABSTRACT

A sub-cooler type heat exchanger assembly and system for use in a cryogenic air separation plant is provided. The sub-cooler type heat exchanger includes at least two separate heat exchange segments within the same housing or shell and is configured to concurrently cool two or more upward flowing cryogenic liquids using nitrogen-rich streams from the lower pressure distillation column.

TECHNICAL FIELD

The present invention is related to a heat exchange system and assembly for use in a cryogenic air separation plant, and more particularly, to a sub-cooler assembly comprising at least two separate heat exchange segments within the same housing or shell and configured to concurrently cool two or more upward flowing cryogenic liquids using nitrogen-rich streams from the lower pressure distillation column.

BACKGROUND OF THE INVENTION

In a typical air separation unit, saturated kettle liquid and shelf liquid from the higher pressure distillation column are sub-cooled in a heat exchanger against a nitrogen stream from the lower pressure distillation column (lower pressure column) before the sub-cooled streams are sent to the lower pressure distillation column. Sub-cooling the kettle liquid and shelf liquid streams prior to introduction into to the lower pressure distillation column tends to minimize flashing of such liquid streams in the column, thereby maximizing liquid reflux in the lower pressure column which enhances the recovery of oxygen product and argon product. In addition, sub-cooling of the kettle liquid and shelf liquid streams aids in the recovery of refrigeration from the nitrogen streams, namely the nitrogen product stream and/or the waste nitrogen reducing the external refrigeration requirements for the air separation plant. Sub-cooling the kettle liquid and shelf liquid streams is preferably targeted at temperatures very close to the temperatures of nitrogen product stream and/or the waste nitrogen stream in order to recover most of the refrigeration and maximize refrigeration recovery from the nitrogen streams.

Usually, such exchange of heat between the nitrogen streams from the lower pressure column and the kettle liquid and shelf liquid streams from the higher pressure column is carried out using a Brazed Aluminum Heat Exchanger (BAHX), commonly referred to as a sub-cooler. This sub-cooler could be a separate, stand-alone heat exchanger or may be packaged within the primary heat exchanger shell and integrated therewith.

Both the external sub-cooler and the integrated sub-cooler have selected advantages and shortcomings. For example, an external or separate sub-cooler typically would involve higher capital costs as well as packaging challenges and may also result in higher pressure loss or pressure drops of the cooling nitrogen streams. However, the external or separate sub-cooler typically offers more design flexibility in terms of selection of the quantity, dimensions, and number of layers, and flow direction for each stream traversing through the sub-cooler.

On the other hand, an integrated sub-cooler typically has the advantage of lower capital costs, lower pressure drops and easier or more simplified packaging. Disadvantages of the integrated sub-cooler is the reduced design flexibility as most of the sub-cooler design parameters are dictated or fixed by the design decisions associated with the primary heat exchanger. An example of an integrated sub-cooler is disclosed in U.S. Pat. No. 6,044,902.

Use of these prior art integrated sub-coolers in heat exchange systems within large-scale, higher pressure air separation units has also resulted in creation of inactive zones within the primary heat exchanger, where little or no heat exchange takes place. The presence of inactive zones at any place where liquid oxygen is present poses a potential safety risk, which is typically mitigated by forcing an active stream to flow through such zones without taking advantage of effective heat exchange, resulting in performance and cost disadvantages. Also, the sub-cooler designs and associated liquid stream flow directions within such integral sub-coolers is often dictated by the design specifications for the primary heat exchanger resulting in inflexible and ineffective sub-cooler design, which tend to make integrated sub-cooler designs more expensive than separate, stand-alone sub-cooler heat transfer assemblies.

Some of the root causes of the sub-optimized performance of the conventional sub-coolers include: (i) uneven distribution of flows entering and exiting multiple sub-coolers due to the physical arrangement of the sub-cooler manifolds and associated piping; (ii) flow deviations of kettle liquid and shelf liquid streams from layer to layer within any given sub-cooler due in part to sub-cooler design, low flow velocities and inability of the liquid streams to fill the entire layer volume; and (iii) existence and variability of two phase flows of kettle liquid directed to and within the sub-coolers. As a result of these problems, there appears to be an under utilization of available heat transfer area within the sub-coolers. Such underperformance of the sub-cooler assemblies could adversely impact argon recovery in the air separation unit.

What is needed therefore, is an improved sub-cooler heat transfer assembly and an improved heat transfer system for a cryogenic air separation plant that mitigates the above-identified problems.

SUMMARY OF THE INVENTION

The present invention may be characterized as a heat exchanger assembly for a cryogenic air separation unit, comprising: (i) a sub-cooler housing having a shell, at least two cryogenic liquid inlets, at least two cryogenic liquid outlets, at least one nitrogen-rich stream inlet and at least one nitrogen-rich stream outlet, the housing configured to receive a flow of a nitrogen-rich stream at the nitrogen-rich stream inlet(s) and separate or distinct flows of at least two cryogenic liquids at the cryogenic liquid inlets; (ii) a first heat exchange segment disposed within the housing and configured for receiving a first flow of cryogenic liquid of an air separation unit and for channeling the first flow of cryogenic liquid in a cross flow orientation or a counter-cross flow orientation from the cryogenic liquid inlets to one of the cryogenic liquid outlets; and (iii) a second heat exchange segment unit disposed within the housing and configured for receiving a second flow of cryogenic liquid and for channeling the second flow of cryogenic liquid within the second heat exchange segment from another of the cryogenic liquid inlets to another of the cryogenic liquid outlets. The first heat exchange segment is configured for receiving a portion of the flow of nitrogen-rich stream and for channeling that flow in a first direction within the first heat exchange segment from the nitrogen-rich stream inlet to the nitrogen-rich stream outlet(s) to sub-cool the first flow and wherein the first direction is generally orthogonal to the first flow of the cryogenic liquid. The second heat exchange segment is further configured for receiving a portion of the flow of the nitrogen-rich stream and for channeling the flow in a second direction within the second heat exchange segment from the nitrogen-rich stream inlet to the nitrogen-rich stream outlets to sub-cool the second flow of the cryogenic liquid.

The present invention may also be characterized as a heat exchanger system for an air separation unit, comprising: (a) a primary heat exchanger having a plurality of heat exchanging units, the primary heat exchanger configured to cool a compressed and purified incoming air stream to temperatures suitable for cryogenic rectification of the air stream in distillation columns via indirect heat exchange with return streams from the air separation unit; and (b) a sub-cooling heat exchanger fluidically coupled to one or more of the heat exchanging units in the primary heat exchanger, the sub-cooling heat exchanger configured to sub-cool at least two cryogenic liquid streams via indirect heat exchange with a nitrogen-rich stream selected from the group comprising a waste nitrogen stream, a product nitrogen stream, or other nitrogen-containing return stream, wherein the sub-cooling heat exchanger is separated and disposed apart from the primary heat exchanger.

Within the present heat exchanger system, the sub-cooling heat exchanger further comprises: (i) a sub-cooler housing having a shell, at least two cryogenic liquid inlets, at least two cryogenic liquid outlets, at least one nitrogen-rich stream inlet and at least one nitrogen-rich stream outlet, the housing configured to receive a flow of the nitrogen-rich stream at the nitrogen-rich stream inlet and separate flows of at least two cryogenic liquids at the cryogenic liquid inlets; (ii) a first heat exchange segment disposed within the housing and configured for receiving a first flow of cryogenic liquid and for channeling the first flow of the cryogenic liquid in a cross flow orientation or a counter-cross flow orientation from one of the cryogenic liquid inlets to one of the cryogenic liquid outlets; the first heat exchange segment further configured for receiving a portion of the nitrogen-rich stream and for channeling the portion of the nitrogen-rich stream in a first direction within the first heat exchange segment from the nitrogen-rich stream inlet to the nitrogen-rich stream outlet to sub-cool the first flow of cryogenic liquid and wherein the first direction is generally orthogonal to the first flow of cryogenic liquid; and (iii) a second heat exchange segment unit disposed within the housing and configured for receiving a second flow of cryogenic liquid and for channeling the second flow of cryogenic liquid within the second heat exchange segment from another of the cryogenic liquid inlets to another of the cryogenic liquid outlets; the second heat exchange segment further configured for receiving a portion of the nitrogen-rich stream and for channeling the portion of the nitrogen-rich stream in a second direction within the second heat exchange segment from the nitrogen-rich stream inlet to the nitrogen-rich stream outlet to sub-cool the second flow of the cryogenic liquid.

The heat exchanger system also may include or comprise an inlet manifold disposed upstream of the sub-cooling heat exchanger and fluidically coupled thereto and configured to deliver the nitrogen-rich stream from a lower pressure distillation column of the air separation unit to the at least one nitrogen-rich stream inlet. In addition, the heat exchange system may include one or more exhaust manifolds disposed downstream of the sub-cooling heat exchanger and fluidically coupled thereto, the one or more exhaust manifolds configured to deliver the warmed nitrogen-rich stream from the sub-cooling heat exchanger to one or more of the heat exchanging units of the primary heat exchanger as a portion of the return streams.

Additional features and elements associated with the present inventions may include arrangements where the first flow of cryogenic liquid and second flow of cryogenic liquid may comprise one or more of the following flows: a flow of kettle liquid from the higher pressure column; a flow of shelf liquid from the higher pressure column; a flow of liquid oxygen; or a flow of liquid air. Advantageously, some or all of the first flow and/or second flow of cryogenic liquids within the sub-cooler assembly flow in a cross-counter flow or serpentine path and in a generally upward flow orientation through the sub-cooler assembly.

The nitrogen-rich stream may comprise waste nitrogen and/or product nitrogen from the lower pressure column of the air separation unit and the flow of the nitrogen-rich stream is a gravity assisted flow in a generally downward orientation. The flow of the cryogenic liquids may be in a counter flow or cross-counter flow path generally orthogonal to the flow of the nitrogen-rich stream. Alternatively, flows of the cryogenic liquids may be in a direction generally parallel to and counter to flow of the nitrogen-rich stream. Still other embodiments of the heat exchanger assembly contemplate splitting the nitrogen-rich stream into two or more exit streams.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the present invention will be more apparent from the following, more detailed description thereof, presented in conjunction with the following drawings, in which:

FIG. 1 shows a general schematic illustration of a portion of a heat exchange system within a cryogenic air separation unit in accordance with the present invention;

FIGS. 2A and 2B show an exterior planar view of the sub-cooler type heat exchanger assembly in accordance with one embodiment of the present invention;

FIGS. 3A and 3B show a cross sectional view of the sub-cooler type heat exchanger assembly depicting the individual heat exchange layers and internal flow paths of the cryogenic liquids and nitrogen-rich stream in accordance with the embodiment of FIG. 2A;

FIGS. 4A and 4B show a cross sectional view of the sub-cooler type heat exchanger assembly depicting an alternate arrangement of individual heat exchange layers and internal flow paths of the cryogenic liquids and nitrogen-rich stream;

FIGS. 5A and 5B show an exterior planar view of the sub-cooler type heat exchanger assembly in accordance with an alternate embodiment of the present invention;

FIGS. 6A and 6B show a cross sectional view of the sub-cooler type heat exchanger assembly depicting the individual heat exchange layers and internal flow paths of the cryogenic liquids and nitrogen-rich streams in accordance with the embodiment of FIGS. 5A; and

FIGS. 7A and 7B show a cross sectional view of the sub-cooler type heat exchanger assembly depicting an alternate arrangement of individual heat exchange layers and internal flow paths of the cryogenic liquids and nitrogen-rich streams.

For the sake of avoiding repetition, some of the common elements in the various Figures utilize the same numbers.

DETAILED DESCRIPTION

Turning now to FIG. 1, there is shown a general schematic illustration of a portion of a heat exchanger system 10 within a cryogenic air separation unit. The illustrated heat exchanger system 10 includes a primary heat exchanger 12 having a plurality of heat exchanging units 14 and one or more sub-cooling heat exchanger assemblies 20 coupled to one or more of the heat exchanging units 14 in the primary heat exchanger 12.

The illustrated embodiment also includes a first inlet manifold or conduit 15 disposed upstream and coupled to the sub-cooling heat exchanger assembly 20. This inlet conduit 15 is configured to deliver a stream of waste nitrogen from the lower pressure distillation column of the air separation unit to one of the sub-cooling heat exchanger assemblies 20. A second inlet manifold or conduit 17 is configured to deliver a stream of nitrogen product from the lower pressure distillation column of the air separation unit to another sub-cooling heat exchanger assembly 20. The waste nitrogen stream and nitrogen product stream are used to sub-cool one or more cryogenic liquids within the sub-cooler heat exchanger assembly 20. Although not shown, the cryogenic liquids may be selected from the one or more of the following streams: a kettle liquid stream, a shelf liquid stream, liquid air stream and liquid oxygen stream. The present system further includes one or more exhaust manifolds 16, 18, 19 coupled to the sub-cooling heat exchanger assemblies 20. The one or more exhaust manifolds 16, 18, 19 are configured to deliver the effluent waste nitrogen stream or the effluent nitrogen product stream from the sub-cooling heat exchanger assembly 20 to one or more of the heat exchanging units 14 of the primary heat exchanger 12 as a portion of the return streams. The primary heat exchanger 12 receives these effluent streams and uses excess refrigeration in such effluent streams to cool a compressed and purified incoming air stream to temperatures suitable for cryogenic rectification of the air stream in distillation columns.

Turning to FIGS. 2A and 2B there is shown exterior planar views of the sub-cooler type heat exchanger assembly 20. As seen therein, the sub-cooler type heat exchanger assembly 20 includes a housing or shell 22 containing a main heat exchange body 30 that is split into two separate heat exchange segments 32, 34 and separated by a divider 35. While the divider 35 is shown at the midpoint of the main heat exchange body 30 such that the two heat exchange segments 32, 34 are of generally equal width, the actual location of the divider 35 may be altered to vary the widths of the two heat exchange segments 32, 34 depending on the cooling requirements for each heat exchange segment within the sub-cooler type heat exchanger assembly 20. Adjusting the location of the divider 35 and thus the widths and spatial volumes of the heat exchange segments 32, 34 provides enhanced design flexibility which is particularly useful in applications requiring the sub-cooling of liquid oxygen from the lower pressure column or liquid air. In addition, by adjusting the widths of the heat exchange segments 32, 34 one can reduce the pressure drop across the sub-cooler type heat exchanger assembly 20.

The illustrated sub-cooler type heat exchanger assembly 20 includes a nitrogen-rich stream inlet 24, two or more nitrogen-rich stream outlets 26, a full dome inlet distributing manifold 27 configured to receive the nitrogen-rich stream (shown as arrow C), and a full dome collection manifold 29 configured to collect the nitrogen-rich stream (C) from within the heat exchanger assembly 20 and distribute the nitrogen-rich stream (C) to the nitrogen-rich stream outlets 26. A flow splitter (not shown) may optionally be disposed within the full dome outlet header or collection manifold 29 to evenly distribute the effluent nitrogen-rich streams (C) to the plurality of nitrogen-rich stream outlets 26.

The illustrated sub-cooler type heat exchanger assembly 20 further includes a plurality of cryogenic liquid inlets and cryogenic liquid outlets. In the illustrated embodiment, the first heat exchange segment 32 includes a cryogenic inlet 42 for a first cryogenic liquid flow (shown as arrow A) and a cryogenic inlet 44 for a second cryogenic liquid flow (shown as arrow B). The first heat exchange segment 32 also includes a cryogenic outlet 46 for the first cryogenic liquid flow (A) and a cryogenic outlet 48 for the second cryogenic liquid flow (B). The second heat exchange segment 34 also includes a cryogenic inlet 52 for another first cryogenic liquid flow (shown as arrow A) and a cryogenic inlet 54 for another second cryogenic liquid flow (shown as arrow B) as well as corresponding cryogenic outlet 56 for the first cryogenic liquid flow (A) and cryogenic outlet 58 for the second cryogenic liquid flow (B). The illustrated design contemplates the first cryogenic liquid flow (A) as being kettle liquid from the high pressure distillation column, and second cryogenic liquid flow (B) as being shelf liquid from the higher pressure column which are sub-cooled against a flow of a waste nitrogen stream (C).

Turning now to FIGS. 3A and 3B, there is shown a cross sectional view of an embodiment of the main heat exchanger body 30 depicting the individual heat exchange layers 40A, 40B and internal flow paths of the cryogenic liquids and nitrogen-rich stream. As in many heat exchangers, there are a plurality of each type of layers. In the present embodiment of the sub-cooler type heat exchanger assembly 20 there are 66 layers of the type shown in FIG. 3A and a corresponding number of layers or more of the of the type shown in FIG. 3B arranged in an alternating pattern or sandwich pattern. The minimum number of FIG. 3A type layers 40A required to ensure complete filling of the passages 45, 47, 55, 57 with kettle liquid and shelf liquid could be determined and implemented independently of number of FIG. 3B type layers 40B for the nitrogen-rich streams required to achieve the desired sub-cooling of the cryogenic liquid streams.

As illustrated, the flow of the nitrogen-rich stream (C) within each FIG. 3B type layer 40B is a gravity assisted flow in a generally downward orientation from the nitrogen-rich stream inlet 62 to one or more nitrogen-rich stream outlets 66. Conversely, the flows of the kettle liquid (A) and shelf liquid (B) within each FIG. 3A type layer 40A are against gravity in a generally upward orientation through the heat exchanger main body 30 in both heat exchanger segments 32, 34. Note the cryogenic liquid inlets 42, 44, 52, and 54 are disposed vertically below the corresponding cryogenic liquid outlets 46, 48, 56, and 58 such that the overall flow of the cryogenic liquids is in an upward flow orientation in both heat exchanger segments 32, 34. In addition, the cryogenic liquid passages 45, 47 within each of the FIG. 3A type layer 40A in the first heat exchange segment 32 and the passages 55, 57 within each of the FIG. 3A type layer 40A in the second heat exchange segment 34 are configured in a cross-counter or serpentine path orthogonal to the path of the nitrogen-rich stream (C) in the adjacent FIG. 3B type layers 40B. Perforated fins are preferably used with the serpentine flow paths to effect the transfer of heat.

FIGS. 4A and 4B show an alternate arrangement of individual heat exchange layers 40A, 40B and internal flow paths of the cryogenic liquids and nitrogen-rich stream. In this embodiment, the first heat exchange segment 32 includes a cryogenic inlet 42 for the first cryogenic liquid flow (shown as arrow A) and a cryogenic inlet 44 for the second cryogenic liquid flow (shown as arrow B). As with the embodiment of FIG. 3A, the first heat exchange segment 32 also includes a cryogenic outlet 46 for the first cryogenic liquid flow (A) and a cryogenic outlet 48 for the second cryogenic liquid flow (B). The second heat exchange segment 34 however, includes a cryogenic inlet 52 for a cryogenic liquid flow (shown as arrow E) and a corresponding cryogenic outlet 56 for the cryogenic liquid flow (E). The illustrated design contemplates the first cryogenic liquid flow (A) as being kettle liquid from the high pressure distillation column, the second cryogenic liquid flow (B) as being shelf liquid from the higher pressure column, and the cryogenic liquid flow (E) as being liquid oxygen from the lower pressure distillation column, all of which are sub-cooled against a flow of waste nitrogen stream (C). Passages 65 within each of the FIG. 4A type layer 40A in the second heat exchange segment 34 are configured in a counter flow arrangement in a direction generally parallel to the path of the nitrogen-rich stream (C) in the adjacent FIG. 4B type layers 40B. Perforated fins or hardway fins are preferably used with the counter flow paths to effect the required heat transfer.

FIGS. 5A and 5B show exterior planar views of an alternate embodiment of the sub-cooler type heat exchanger assembly 20. Similar to the embodiment of FIGS. 2A and 2B, the illustrated heat exchanger assembly 20 includes housing or shell 22 and a main heat exchange body 30 that is split into two separate heat exchange segments 32, 34 separated by a divider 35. The illustrated sub-cooler type heat exchanger assembly also includes two nitrogen-rich stream inlets 23, 24; two or more nitrogen-rich stream outlets 25, 26; split dome inlet distributing manifolds 27 configured to receive flows of the nitrogen-rich streams (shown as arrow C and arrow D), and split dome collection manifolds 29 configured to collect the flows of nitrogen-rich streams (C), (D) from within the heat exchanger assembly 20 and distribute the nitrogen-rich streams to the corresponding nitrogen-rich stream outlets 25, 26.

The illustrated sub-cooler type heat exchanger assembly 20 further includes a plurality of cryogenic liquid inlets and cryogenic liquid outlets. In the illustrated embodiment, the first heat exchange segment 32 includes a cryogenic inlet 42 for the first cryogenic liquid flow (shown as arrow A) and a cryogenic inlet 44 for the second cryogenic liquid flow (shown as arrow B). The first heat exchange segment 32 also includes a cryogenic outlet 46 for the first cryogenic liquid flow (A) and a cryogenic outlet 48 for the second cryogenic liquid flow (B). The second heat exchange segment 34 includes a cryogenic inlet 52 for a cryogenic liquid flow (shown as arrows E and/or F) as well as a corresponding cryogenic outlet 56 for the cryogenic liquid flow (E/F). The illustrated design contemplates the first cryogenic liquid (A) as being kettle liquid and the second cryogenic liquid (B) as being shelf liquid which are sub-cooled against a waste nitrogen stream (C). In addition, cryogenic liquid flow (E/F) is either a flow of liquid oxygen from the lower pressure column or a flow of liquid air which are sub-cooled against a nitrogen product stream (D).

Turning now to FIGS. 6A and 6B, there is shown a cross sectional view of the embodiment of the main heat exchanger body 30 depicting the individual heat exchange layers 40A, 40B and internal flow paths of the cryogenic liquids and nitrogen-rich streams. As illustrated, the flows of the waste nitrogen stream (C) and product nitrogen stream (D) within each FIG. 6B type layer 40B is a gravity assisted flow in a generally downward orientation through their respective heat exchanger segments 32, 34. Conversely, the flow of the kettle liquid (A) and shelf liquid (B) within each FIG. 6A type layer 40A and the flow of cryogenic liquid (E/F) are against gravity in a generally upward orientation through the heat exchanger main body 30 in their respective heat exchanger segments 32, 34. As discussed above, the cryogenic liquid inlets 42, 44, and 52 are disposed vertically below the corresponding cryogenic liquid outlets 46, 48, and 56 such that the overall flow of the cryogenic liquids is in an upward flow orientation in the heat exchanger assembly 20. Similar to the embodiment of FIG. 4A, the cryogenic liquid passages 45, 47 within each of the FIG. 6A type layer 40A in the first heat exchange segment 32 are configured in a cross-counter or serpentine path orthogonal to the path of the waste nitrogen stream (C) in the adjacent FIG. 3B type layers 40B. Passages 65 within each of the FIG. 6A type layers 40A in the second heat exchange segment 34 are configured in a counter flow arrangement generally parallel to the path of the nitrogen product stream (D) in the adjacent FIG. 6B type layers 40B.

FIGS. 7A and 7B show yet another alternate arrangement of individual heat exchange layers and internal flow paths of the cryogenic liquids and nitrogen-rich streams. Again, the flows of the waste nitrogen stream (C) and product nitrogen stream (D) within each FIG. 7B type layer 40B is a gravity assisted flow in a generally downward orientation through their respective heat exchanger segments 32, 34 while the cryogenic flows (A), (B), (E) and (F) are against gravity in a generally upward orientation through the heat exchanger main body in their respective heat exchanger segments 32, 34. In this embodiment, the cryogenic liquid passages 45, 47, 55, 57 within each of the FIG. 7A type layers 40A in the heat exchange segments 32, 34 are configured in a cross-counter or serpentine path generally orthogonal to the path of the nitrogen-rich streams (C), (D) in the adjacent FIG. 7B type layers 40B.

The present heat exchange system described above provides some power savings for the air separation unit through reduced pressure drops in the present sub-cooler type heat exchanger assembly compared to conventional integrated sub-cooler assemblies as well as conventional separate sub-cooling assemblies. In addition, the present heat exchange system can realize potential sub-cooling performance enhancements due to elimination of inactive zones in a primary heat exchanger with integrated sub-cooler assemblies and utilization of the full heat transfer area within the present sub-cooler heat transfer assemblies. Such sub-cooling performance enhancements translate into improved argon recovery within the air separation unit.

The present heat exchange system also provides some capital cost savings associated with the primary heat exchanger compared to conventional primary heat exchangers with an integrated sub-cooler. The reduction in capital costs is partially offset by the added equipment costs for the separate sub-cooler heat exchange assemblies. Specifically, the capital cost savings associated with reduction in primary heat exchanger length and complexity.

While the present invention has been described with reference to selected preferred embodiments, numerous additions, modifications, and variances can be made without departing from the spirit and scope of the present invention as set forth in the appended claims. 

What is claimed is:
 1. A heat exchanger assembly for a cryogenic air separation unit, comprising: a housing having a shell, at least two cryogenic liquid inlets, at least two cryogenic liquid outlets, at least one nitrogen-rich stream inlet and at least one nitrogen-rich stream outlet, the housing configured to receive a flow of at least one nitrogen-rich stream of the air separation unit at the at least one nitrogen-rich stream inlet and separate flows of at least two cryogenic liquids at the at least two cryogenic liquid inlets; a first heat exchange segment disposed within the housing and configured for receiving a first flow of at least one cryogenic liquid of an air separation unit and for channeling the first flow of the at least one cryogenic liquid in a cross flow orientation or a counter-cross flow orientation from at least one of the cryogenic liquid inlets to at least one of the cryogenic liquid outlets; the first heat exchange segment further configured for receiving a portion of the flow of the at least one nitrogen-rich stream and for channeling a portion of the flow of the at least one nitrogen-rich stream in a first direction within the first heat exchange segment from the at least one nitrogen-rich stream inlet to the at least one nitrogen-rich stream outlet to sub-cool the first flow of the at least one cryogenic liquid and wherein the first direction is generally orthogonal to the first flow of the at least one cryogenic liquid; a second heat exchange segment unit disposed within the housing and configured for receiving a second flow of at least one cryogenic liquid of an air separation unit and for channeling the second flow of the at least one cryogenic liquid within the second heat exchange segment from another of the cryogenic liquid inlets to another of the cryogenic liquid outlets; and the second heat exchange segment further configured for receiving a portion of the flow of the at least one nitrogen-rich stream and for channeling the portion of the flow of the at least one nitrogen-rich stream in a second direction within the second heat exchange segment from the at least one nitrogen-rich stream inlet to the at least one nitrogen-rich stream outlet to sub-cool the second flow of the at least one cryogenic liquid.
 2. The heat exchanger assembly of claim 1 wherein the housing further comprises at least two nitrogen-rich stream outlets and the flow of at least one nitrogen-rich stream exiting the housing is split into at least two streams.
 3. The heat exchanger assembly of claim 1 wherein the first flow of at least one cryogenic liquid comprises a flow of kettle liquid from the higher pressure column.
 4. The heat exchanger assembly of claim 1 wherein the first flow of at least one cryogenic liquid comprises a flow of shelf liquid from the higher pressure column.
 5. The heat exchanger assembly of claim 1 wherein the first flow of at least one cryogenic liquid comprises a flow of liquid oxygen from the lower pressure column.
 6. The heat exchanger assembly of claim 1 wherein the first flow of at least one cryogenic liquid comprises multiple flows of cryogenic liquids.
 7. The heat exchanger assembly of claim 6 wherein the multiple flows are selected from the group consisting essentially of a flow of kettle liquid from the higher pressure column; a flow of shelf liquid from the higher pressure column; a flow of liquid oxygen; and a flow of liquid air.
 8. The heat exchanger assembly of claim 1 wherein the second flow of at least one cryogenic liquid comprises a flow of kettle liquid from the higher pressure column and wherein the second direction is generally orthogonal to the second flow.
 9. The heat exchanger assembly of claim 1 wherein the second flow of at least one cryogenic liquid comprises a flow of shelf liquid from the higher pressure column and wherein the second direction is generally orthogonal to the second flow.
 10. The heat exchanger assembly of claim 1 wherein the second flow of at least one cryogenic liquid comprises a flow of liquid oxygen the lower pressure column and wherein the second direction is generally orthogonal to the second flow.
 11. The heat exchanger assembly of claim 1 wherein the flow of at least one nitrogen-rich stream in the second direction is a gravity assisted flow in a downward orientation.
 12. The heat exchanger assembly of claim 11 wherein the flow of the at least one nitrogen-rich stream further comprise waste nitrogen or product nitrogen or both from a lower pressure column of the air separation unit.
 13. The heat exchanger assembly of claim 11 wherein the second flow of at least one cryogenic liquid comprises liquid air and wherein the second direction is generally parallel to the second flow.
 14. The heat exchanger assembly of claim 11 wherein the second flow of at least one cryogenic liquid comprises liquid air and wherein the second direction is in a counter-flow orientation with respect to the second flow.
 15. The heat exchanger assembly of claim 1 wherein the cryogenic liquid inlets are disposed vertically below the corresponding cryogenic liquid outlets such that the overall flow of the cryogenic liquids is in an upward flow orientation.
 16. The heat exchanger assembly of claim 1 wherein the spatial volume of the first heat exchange segment within the housing and the spatial volume of the second heat exchange segment within the housing are substantially equal.
 17. The heat exchanger assembly of claim 1 wherein the width or spatial volume of the first heat exchange segment within the housing and the width or spatial volume of the second heat exchange segment within the housing are different.
 18. A heat exchanger system for an air separation unit, comprising: a primary heat exchanger having a plurality of heat exchanging units, the primary heat exchanger configured to cool a compressed and purified incoming air stream to temperatures suitable for cryogenic rectification of the air stream in distillation columns via indirect heat exchange with return streams from the air separation unit; and a sub-cooling heat exchanger fluidically coupled to one or more of the heat exchanging units in the primary heat exchanger, the sub-cooling heat exchanger configured to sub-cool at least two cryogenic liquid streams via indirect heat exchange with a nitrogen-rich stream selected from the group comprising a waste nitrogen stream, a product nitrogen stream, or other nitrogen-rich return stream, the sub-cooling heat exchanger comprising: (i) a housing having a shell, at least two cryogenic liquid inlets, at least two cryogenic liquid outlets, at least one nitrogen-rich stream inlet and at least one nitrogen-rich stream outlet, the housing configured to receive a flow of the nitrogen-rich stream at the at least one nitrogen-rich stream inlet and separate flows of at least two cryogenic liquids at the at least two cryogenic liquid inlets; (ii) a first heat exchange segment disposed within the housing and configured for receiving a first flow of cryogenic liquid and for channeling the first flow of the cryogenic liquid in a cross flow orientation or a counter-cross flow orientation from at least one of the cryogenic liquid inlets to at least one of the cryogenic liquid outlets; the first heat exchange segment further configured for receiving a portion of the nitrogen-rich stream and for channeling the portion of the nitrogen-rich stream in a first direction within the first heat exchange segment from the at least one nitrogen-rich stream inlet to the at least one nitrogen-rich stream outlet to sub-cool the first flow of cryogenic liquid and wherein the first direction is generally orthogonal to the first flow of cryogenic liquid; and (iii) a second heat exchange segment unit disposed within the housing and configured for receiving a second flow of cryogenic liquid and for channeling the second flow of cryogenic liquid within the second heat exchange segment from another of the cryogenic liquid inlets to another of the cryogenic liquid outlets; the second heat exchange segment further configured for receiving a portion of the nitrogen-rich stream and for channeling the portion of the nitrogen-rich stream in a second direction within the second heat exchange segment from the at least one nitrogen-rich stream inlet to the at least one nitrogen-rich stream outlet to sub-cool the second flow of the cryogenic liquid; wherein the sub-cooling heat exchanger is separated and disposed apart from the primary heat exchanger.
 19. The heat exchanger system of claim 18 further comprising: an inlet manifold disposed upstream of the sub-cooling heat exchanger and fluidically coupled thereto and configured to deliver the nitrogen-rich stream from a lower pressure distillation column of the air separation unit to the at least one nitrogen-rich stream inlet; and one or more exhaust manifolds disposed downstream of the sub-cooling heat exchanger and fluidically coupled thereto, the one or more exhaust manifolds configured to deliver the warmed nitrogen-rich stream from the sub-cooling heat exchanger to one or more of the heat exchanging units of the primary heat exchanger as a portion of the return streams. 